1. Field of the Invention
The present invention relates to an optical pickup apparatus configured to guide first laser light and second laser light having different wavelengths to an objective lens by refracting optical axes by a reflection mirror through a common optical path and to apply the laser light to an optical recording medium, in which the first laser light and the second laser light, which are linear polarization emitted from each of a first laser light source and a second laser light source, are applied to the recording medium as the first laser light and the second laser light of circular polarization.
2. Description of the Related Art
In an optical pickup apparatus used for optically recording/reproducing signals with the use of laser light to an optical recording medium such as an optical disc including DVD (Digital Versatile Disc) and CD (Compact Disc), there is known an optical pickup apparatus which supports DVD and CD with different recording densities with a single optical pickup apparatus.
Such an optical pickup apparatus compatible with DVD and CD includes a first laser light source emitting laser light of 645 nm to 675 nm in a red wavelength band suitable for DVD and also includes a second laser light source emitting laser light of 765 nm to 805 nm in an infrared wavelength band suitable for CD, and the laser light to be used is switched according to an optical disc.
The first laser light source and the second laser light source generally include a semiconductor laser of laser diode, respectively, and a multi laser unit is often employed in which each laser light source includes a single laser chip, or laser light sources include respective laser chips and are installed on the same semiconductor base so as to be incorporated in a single package.
Also, in the optical pickup apparatus compatible with DVD and CD, an objective lens in which an annular diffraction grating is formed on an incident face is used, and a quality of each laser light applied to each optical disc is ensured by correcting spherical aberration with respect to each optical disc of DVD and CD by diffracting each laser light having a wavelength suitable for each optical disc with this diffraction grating so as to correspond to DVD and CD by a single objective lens.
The optical pickup apparatus compatible with DVD and CD has its optical path simplified by employing the above-mentioned multi laser unit and a single objective lens.
In an optical pickup apparatus, in order to alleviate a bad influence caused by double refraction of a transparent substrate resin of the optical disc, laser light applied onto a signal layer of the optical disc is made circular polarization.
A semiconductor laser, which is a laser light source of the optical pickup apparatus used for recording/reproduction of the optical disc, emits laser light of linear polarization due to its structure, so that the optical pickup apparatus needs a quarter-wave plate for generating a phase difference of quarter-wave to the laser light emitted from the laser light source in order to make the laser light to be applied to the optical disc circular polarization.
Also, since the optical pickup apparatus can make a linear polarization directions of laser light different by 90 degrees between a outward path for guiding the laser light from the laser light source to the optical disc and a return path for guiding the laser light reflected by the optical disc to a photodetector by a quarter-wave plate, the laser light source and the photodetector can be arranged in different optical paths with high utilization efficiency by using a polarizing beam splitter.
In an optical pickup apparatus in which two sets of laser light sources with different wavelengths compatible with DVD and CD are provided, and each laser light emitted from each laser light source is guided to a common optical path and made to correspond to each optical disc through a common objective lens, there is known a configuration that a reflection mirror for raising arranged in front of the objective lens on the outward path is given a function of the quarter-wave plate, and laser light with each wavelength compatible with DVD and CD is made circular polarization and applied to the optical disc (See Japanese Patent Application Laid-Open Publication No. 2008-251112).
In the optical pickup apparatus disclosed in Japanese Patent Application Laid-Open Publication No. 2008-251112, since the raising reflection mirror is given the function of the quarter-wave plate, the number of optical components is decreased, which is advantageous in the number of assembling processes and miniaturization, but a phase difference of a quarter-wave needs to be generated to the laser light with each wavelength compatible with DVD and CD by the raising reflection mirror. Thus, the raising reflection mirror needs to be given a phase difference function of a quarter-wave plate in a wide band corresponding to the laser light with each wavelength compatible with DVD and CD. Since it is difficult to form a reflective film capable of both reflectivity of the reflection mirror and the phase difference function or it is necessary to provide a phase difference film of the quarter-wave plate in a wide band through bonding not by coating on a reflection surface of the reflection mirror, it is disadvantageous in terms of cost.